The proposed research program is aimed at investigating three areas of cognitive development: children's existing knowledge, their ability to acquire new knowledge, and the more basic processes that may underlie developmental changes in the first two domains. These issues will be explored with a wide age range of children, ranging from three-years through college age. Past research indicates that many of the scientific reasoning tasks under investigation have a desirable property for developmental research; even very young children know something about the tasks, while even adults do not know everything there is to know. Thus, the problems will allow us to observe development over an unusually protracted time period. A variety of theoretical perspectives will be brought to bear on the issues described above. The proposed experiments are guided by ideas derived from Tragetian, information processing, and traditional learning models. All of these theories have contributed both to the content of the present approach and to the methodologies that will be used.